Green water is one of the most common aquarium algae problems. What it is: a bloom of free-floating single-celled algae (mostly Chlorella + Chlamydomonas). The water turns from clear to pea-soup green over 3-7 days. Causes: 1) Direct sunlight on tank - the #1 trigger. Move tank or close curtains. 2) Excess nutrients (high nitrate + phosphate) - cut feeding 50%. 3) Long photoperiod (10+ hours) - reduce to 6-8 hours. 4) Spike from disturbed substrate - algae spores released. 5) New tank "diatom phase" sometimes turns green if silicates run out. Solutions: 1) **3-day blackout** - cover tank entirely (no light at all), turn off light, fish okay for 3 days, cover for 4 days = algae dies + settles. Restart light at 6 hours. 2) **UV sterilizer** - 9-13W kills algae passing through. Clears tank overnight. Most reliable. 3) **Fast-growing stem plants** - hornwort, water sprite, frogbit consume nitrate + phosphate before algae can. 4) **Daphnia introduction** - filter feeders eat algae cells. 5) **Reduce feeding + larger water changes** - less nutrient input. Avoid: chemical "algae killers" (copper-based, kills inverts) + flocculants (just precipitates algae temporarily).
The full Q&A library, calculators, diseases, glossary, and interactive tools answer the most-searched aquarium questions.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.
How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.
What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.
How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.
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